1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
Japanese Patent Application Publication No. 2010-116090 (JP 2010-116090 A) describes a worm reducer that includes inter-axis distance reduction urging means and lead angle correction urging means, as urging means for urging a bearing that supports a worm shaft toward a bearing hole portion. The inter-axis distance reduction urging means reduces the inter-axis distance between the worm shaft and a worm wheel. The lead angle correction urging means urges the bearing in such a direction that the lead angle of a worm is larger than the lead angle of the worm wheel.
According to JP 2010-116090 A, the bearing hole needs to be formed into a shape that allows the bearing to be displaced in two directions, that is, in such a direction that the inter-axis distance is increased or decreased to adjust a backlash (a clearance between tooth flanks) and such a direction that the lead angle of the worm is increased to increase the contact ratio between the tooth flanks. Therefore, it is difficult to form the bearing hole. In addition, actually, the worm shaft is urged in the direction of a resultant force of an urging force that is generated by the inter-axis distance reduction urging means and an urging force that is generated by the lead angle correction urging means. Therefore, there is a possibility that the backlash will not be accurately adjusted due to the influence of variations, such as a manufacturing error of each urging means.
The direction in which a worm shaft with a lead angle is rotated for steering to the left is opposite to the direction in which the worm shaft is rotated for steering to the right. Therefore, friction torque between tooth flanks differs between steering to the left and steering to the right. As a result, the steering feel differs between steering to the left and steering to the right.